Operation of Center Vent Vac Pod - W123

[techguy512]

I'm checking the operation of all my vac pods. Most work. Very confused about the proper operation of the center vent pod.

My understanding is that this is a single element pod, meaning that vac is applied to move it in one direction, and spring tension moves it in the other. But which is which?

When I manually open the center vent, then switch to 'heat' the system gives a very weak, half-hearted effort to close the center vents. With the vents closed, selecting 'AC' does nothing, and the vents just stay closed.

Should I be looking for vacuum to open the vents in AC mode or close the vents in heat mode, and do I have the proper general understanding of the operation of the center vent vac pod?

Thanks,